The goals of this research are to understand the role of host immunity in the pathogenesis of the major disease manifestations of bancroftian filariasis, why they are distributed heterogeneously in at-risk populations, and how control programs will affect this morbidity. Based on observations made in a highly endemic area of Papua New Guinea over the past 17 years, the three specific aims will: 1. Examine the hypothesis that lymphatic dilatation of the vas deferens and hydroceles are due to establishment of adult worms and not related to antigen-specific host immunity. 2. Determine the importance of host T-cell immunity and related allergic responses m the pathogenesis of ADL and chronic lymphedema of the leg. This aim will evaluate filarial Ag-stimulated T-cell and basophil responses in persons with recurrent ADL and chronic lymphedema of the leg, and compare them with those of asymptomatic individuals matched for age, infection status, and transmission intensity. 3. Evaluate the reversibility of the various types of clinically overt and asymptomatic lymphatic pathology following introduction of mass chemotherapy, and determine whether they correlate with changes in filarial Ag-specific immunity. These studies will provide novel insights into how transmission, immunity, and lymphatic pathology are interrelated, and advance knowledge of how current strategies to control lymphatic filariasis may affect the major types of morbidity associated with this infectious disease.